It is generally accepted, that increasing oil prices and national legislation demand the reduction of automotive carbon dioxide emissions, thus requesting from tire and rubber producers to produce “fuel efficient” tires. One general approach to obtain fuel efficient tires is to produce tire formulations that have reduced hysteresis loss. A major source of hysteresis in vulcanized elastomeric polymers is attributed to free polymer chain ends, that is, the section of the elastomeric polymer chain between the last cross-link and the end of the polymer chain. This free end of the polymer does not participate in any efficient elastically recoverable process, and as a result, energy transmitted to this section of the polymer is lost. This dissipated energy leads to a pronounced hysteresis under dynamic deformation. Another source of hysteresis in vulcanized elastomeric polymers is attributed to an insufficient distribution of filler particles in the vulcanized elastomeric polymer composition. The hysteresis loss of a cross-linked elastomeric polymer composition is related to its Tan δ at 60° C. value (see ISO 4664-1:2005; Rubber, Vulcanized or thermoplastic; Determination of dynamic properties—part 1: General guidance). In general, vulcanized elastomeric polymer compositions having relatively small Tan δ values at 60° C. are preferred as having lower hysteresis loss. In the final tire product, this translates into a lower rolling resistance and better fuel economy.
Furthermore, there are also demands to maintain or improve tire grip properties, particularly the grip of the tire on a wet or icy road. The tire wet and ice grip of a cross-linked elastomeric polymer composition is related to its Tan δ at 0° C. and Tan δ at −10° C. values. It is generally accepted that a lower rolling resistance tire can be made on the expense of deteriorated wet grip properties and vice versa. For example, if, in a random solution styrene-butadiene rubber (random SSBR), the polystyrene unit concentration is reduced with respect to the total polybutadiene unit concentration and the 1,2-polydiene unit concentration is kept constant, both tan delta at 60° C. and tan delta at 0° C. are reduced, generally corresponding to improved rolling resistance and deteriorated wet grip performance of a tire. Similarly, if, in a random solution styrene-butadiene rubber (random SSBR), the 1,2-polybutadiene unit concentration is reduced with respect to the total polybutadiene unit concentration and the polystyrene unit concentration is kept constant, both tan delta at 60° C. and tan delta at 0° C. are reduced, generally corresponding to improved rolling resistance and deteriorated wet grip performance of a tire. Accordingly, when assessing the rubber vulcanizate performance correctly, both the rolling resistance, or tan delta at 60° C., and the wet grip, or tan delta at 0° C., should be monitored.
One generally accepted approach to reducing hysteresis loss is to reduce the number of free chain ends of elastomeric polymers. Various techniques are described in the open literature including the use of “coupling agents,” such as tin tetrachloride, which may functionalize the polymer chain end, and react with components of an elastomeric composition, such as, for example, with a filler or with unsaturated portions of a polymer. Examples of such techniques, along with other documents of interest, are described in the following patents: U.S. Pat. Nos. 3,281,383; 3,244,664 and 3,692,874 (for example, tetrachlorosilane); U.S. Pat. No. 3,978,103; U.S. Pat. Nos. 4,048,206; 4,474,908; U.S. Pat. No. 6,777,569 (blocked mercaptosilancs) and U.S. Pat. No. 3,078,254 (a multi-halogen-substituted hydrocarbon, such as 1,3,5-tri(bromo methyl) benzene); U.S. Pat. No. 4,616,069 (tin compound and organic amino or amine compound); and U.S. 2005/0124740.
The use of “coupling agents” as reactants to living polymers more often than not leads to the formation of polymer blends comprising one fraction of linear or uncoupled polymers, and one or more fractions comprising more than two polymer arms at the coupling point. The reference “Synthesis of end-functionalized polymer by means of living anionic polymerization,” Journal of Macromolecular Chemistry and Physics, 197, (1996), 3135-3148, describes the synthesis of “polystyrene-containing” and “polyisoprene-containing” living polymers with hydroxy (—OH) and mercapto (—SH) functional end caps, obtained by reaction of the living polymers with haloalkanes containing silyl ether and silyl thioether functions. The tertiary-butyldimethylsilyl (TBDMS) group is preferred as a protecting group for the —OH and —SH functions in the termination reactions, because the corresponding silyl ethers and thioethers are found to be both stable and compatible with anionic living polymers.
WO2007/047943 describes the use of a silane-sulfide omega chain end modifier. A silane sulfide compound is reacted with anionically-initiated, living polymers to produce “chain end modified” polymers, which are subsequently blended with fillers, vulcanizing agents, accelerators or oil extenders, to produce a vulcanized elastomeric polymer composition having low hysteresis loss. To further control polymer molecular weight and polymer properties, a coupling agent (or linking agent) can be used as an optional component in the process of the preparation of elastomeric polymers. The modifier is added before, after or during the addition of a coupling agent, and the modification reaction is preferably completed after the addition of the coupling agent. In some embodiments, more than a third of the polymer chain ends are reacted with a coupling agent prior to addition of the modifier.
WO 2009/148932 describes an elastomeric polymer composition as the reaction product of a living anionic elastomeric polymer with two silane modifier compounds (A) and (B). The silane modifier compound (A) is reported to react with at least two polymer chains, forming branched modified polymer macromolecules, while silane modifier compound (B) is reported to react with only one polymer chain, forming chain-end modified polymer macromolecules. The resulting cured composition comprising branched-modified and chain-end modified polymer macromolecules is stated to result in lower “Tan δ at 60° C.” values, without negatively impacting other physical properties, particularly “Tan δ at 0° C.”
WO2007/047943 and WO 2009/148932 do not provide rheological information on filler-containing polymer compositions. Yet, it is reasonable to expect higher viscosities as a result of enhanced polymer-filler associations.
Two fillers, silica and carbon black, are typically used in the tire production. Standard formulations very often comprise both fillers in varying ratios. Therefore, it would be desirable to have a modified polymer (comprising one or both of branched modified polymer macromolecules and chain end modified polymer macromolecules) which exhibits reduced viscosity in (non-cured) polymer compositions, especially lower Mooney viscosity, and/or improved rolling resistance/grip balance characteristics of the cured compositions.